Exam TI2720-C/TI2725-C Embedded Software

Transcription

1 Exam TI2720-C/TI2725-C Embedded Software Wednesday April ( ) Koen Langendoen In order to avoid misunderstanding on the syntactical correctness of code fragments in this examination, we will always assume that we are dealing with pseudo-code, although we might have syntactically correct code in some cases. We assume that the required variables, semaphores, tasks, timers, etc. are always declared and initialized correctly. Further, we assume the following abbreviations to be known: RR = Round Robin, RRI = Round Robin with Interrupts, FQS = Function Queue Scheduling, RTOS = Real-Time Operating System, IR = interrupt and ISR = interrupt service routing. In this exam, we use the following definitions, unless stated otherwise: void delay(int ms) {!! do some CPU computation for ms milliseconds void putchar(char c) { while (!! UART tx buffer not empty);!! send c to UART tx buffer void puts(char *s) {!! print string s using putchar To pass this written exam, you need to correctly answer at least 20 questions. The relationship between the number of correctly answered questions and your mark is given below. #correct Mark #correct Mark

2 Question 1 Embedded programming is more difficult than classical programming because of a. the interaction with hardware b. real-time issues (timing) c. limited visibility (e.g., no display) d. all of the above Question 2 An embedded program can be coded as a finite state machine where a. interrupts trigger state transitions b. user actions trigger all state transitions c. interrupts must be disabled during state transitions d. state transitions occur when RTOS functions are called Question 3 Which of the following statements is true regarding the embedded software crises a. It was triggered by the year 2000 bug b. It was triggered by the increasing speed gap between on- and off-chip communication c. It has been resolved due to Morse law generating ever faster hardware d. It refers to the lack of correct code for the increasing number of embedded systems Question 4 Which of the following statements is correct? An interrupt is a. a synchronous signal form hardware to indicate processor attention b. a synchronous signal from the processor to indicate hardware attention c. an asynchronous signal from hardware to indicate processor attention d. an asynchronous signal from the processor to indicate hardware attention Question 5 Which of the following statements is correct? An interrupt service routine is supposed to a. disable the non-maskable interrupt b. restore the context and return c. restore the lowest-priority interrupt d. increase the program counter Question 6 Which of the following statements is correct? An interrupt vector a. contains a pointer to an ISR b. points to a table with interrupt routines c. contains the addresses of interrupts d. contains interrupt routines Question 7 Which of the following statements is correct? Using interrupts impairs a. task response time b. higher priority ISR response time c. processor response time d. none of the above Question 8 Which of the following statements is correct? a. we cannot have a shared data problem in an RR architecture b. we cannot have a shared data problem in an RRI architecture c. we cannot have interrupt vectors in an FQS architecture d. we cannot have atomic sections in an RTOS architecture Question 9 For which kind of software architecture for embedded systems is the worst case response time for task code equal to the total execution time for all task code plus the execution time for interrupt routines? a. RR b. RRI c. FQS d. RTOS Question 10 Which of the following architectures is most robust to code changes? a. RR b. RRI c. FQS d. RTOS Embedded Software exam April 16,

3 Question 11 In an RTOS, tasks can be in state BLOCKED, READY, or RUNNING. Which of the following statements is true? a. A task can transition directly from BLOCKED to RUNNING b. A task can transition directly from READY to BLOCKED c. A task can transition directly from RUNNING to BLOCKED d. A task can stay in one state for at most one time slice Question 12 After creation a task enters the RTOS in the state a. BLOCKED b. READY c. RUNNING d. that depends on its priority relative to the running task Question 13 Which of the following statements is correct? a. at most one task can be in the state READY b. at most one task can be in the state RUNNING c. only the task with the highest priority can be in the state RUNNING d. only the task with the lowest priority can be in the state BLOCKED Questions Given is the following RTOS (pseudo) code with decreasing priorities T1 > T2 > T3: void f1(void) { delay(100); void f2(void) { delay(100); void f3(void) { delay(100); void T1(void) { OS_Pend(sem1); // wait for event #1 OS_Pend(mutex); delay(100); OS_Post(mutex); f1(); void T2(void) { OS_Pend(sem2); // wait for event #2 f2(); void T3(void) { OS_Pend(sem3); // wait for event #3 OS_Pend(mutex); delay(100); OS_Post(mutex); f3(); Question 14 No other events have been treated by the system previously. The events #1, #2, #3 are triggered periodically, in this order, 10 ms one after another (example: #1,#2,#3,#1,#2,#3,#1,#2,#3,etc.). Which of the following statements is correct? a. f1() is executed at least once b. each of f1() and f2() are executed at least once c. each of f1(), f2(), and f3() are executed at least once d. priority inversion occurs between T1 and T3 Question 15 No other events have been treated by the system previously. Imagine that the following sequence of events comes once: #3, #2, #1 (timing between events is not known). Which of the following statements is always correct? a. f1() is executed first, before f2() and f3() b. f2() is executed first, before f1() and f3() c. f3() is executed first, before f1() and f2() d. we cannot make any prediction on the order of execution Embedded Software exam April 16,

4 Question 16 No other events have been treated by the system previously. Imagine that the following sequence of events comes once: #3, #2, #1 (timing between events is not known). The delay(100) calls are replaced by OSTimeDly(10) calls. Which of the following statements is always correct? a. f1() is executed first, before f2() and f3() b. f2() is executed first, before f1() and f3() c. f3() is executed first, before f1() and f2() d. we cannot make any prediction on the order of execution Question 17 We assume a system clock tick of 10ms. Which of the following statements is most accurate? Calling the function OSTimeDly(3)... a. causes a delay of exactly 30ms b. causes a delay between 20ms and 30ms c. causes a delay between 30ms and 40ms d. causes a delay between 20ms and 40ms Question 18 Semaphores can be used for mutual exclusive access to shared resources. In that case a semaphore must be initialized to a. 0 (zero) b. 1 (one) c. n (number of concurrent tasks) d. m (number of resources) Question 19 In order for priority inversion to occur, it be must be true that a. there are at least two tasks with different priorities b. there are at least three tasks with different priorities c. the RTOS scheduler uses a round-robin policy d. task preemption is disabled Question 20 Mutual exclusive access can also be accomplished by disabling interrupts. Compared to using mutexes this has the advantage that a. context switching is faster b. programs are more responsive to external events c. deadlocks cannot occur d. none of the above Question 21 Which of the following statements is correct? The X32 platform... a. has one interrupt priority level b. has an interrupt controller c. is not equipped with an interrupt vector d. prohibits interrupt preemption Question 22 A re-entrant function f may not use variables in a non-atomic way... a. no matter where they are located b. unless they are allocated on the heap c. unless they are local to f (and allocated on the call stack) d. unless they are declared static in the C implementation file Question 23 Which of the following statements is correct about RTOSs and interrupts? a. an ISR may call any RTOS function because it will never block the caller b. an ISR must not call any RTOS function that might block the caller c. an ISR must not call any RTOS function that might block the caller unless it has a small size d. an ISR may call any RTOS function that might block the caller when it has a small size Question 24 Which of the following statements is correct about RTOS and interrupts? a. an ISR may call any function b. an ISR may only call RTOS functions c. an ISR may never call functions that might cause a context switch d. an ISR may call RTOS functions that might cause a context switch provided that the RTOS knows that an ISR (and not a task) is making the call Embedded Software exam April 16,

5 Question 25 To master the complexity of a program it can be structured as a set of tasks. Which of the following statements is true? a. the memory footprint grows linearly with the number of tasks b. when doubling the number of tasks, the number of semaphores also doubles c. the context switch time between tasks doubles with every new task d. tasks can share the same stack as mutual exclusion allows only one task to execute a critical section Question 26 Creating and destroying tasks dynamically is somewhat problematic because a. the RTOS must disable interrupts for too long b. a task s priority may change during its lifetime c. a task may hold a semaphore when being destroyed d. a task may create another task recursively, draining resources managed by the RTOS Question 27 Running tasks with the same priority complicates an RTOS because a. round robin scheduling requires exclusive access to the heap b. round robin scheduling may lead to oscillating behavior between two high-priority tasks c. response times become unpredictable in overload situations d. the RTOS can no longer access the tasks through a dispatch table Question 28 When developing code for an embedded system, the software can de structured into HW-dependent and HW-independent code. This is done to ensure that a. debugging can be performed by two independent teams of software developers b. debugging HW independent code becomes feasible on the target platform c. debugging HW independent code becomes feasible on the host platform d. debugging HW dependent code becomes feasible during runtime Question 29 One approach to facilitate debugging is to script test scenarios. Which of the following statements is true? a. scripting can only be used to test HW independent code b. a script can only trigger one interrupt at the same time c. each line of a script must start with a time stamp d. parsing a script can easily be done in C Question 30 When debugging code for a distributed system, collecting the (debug) output of the different sensor nodes can be arranged in different ways [Beutel:2009. Which of the following statements is true? a. online sniffing requires logging facilities on the sensor nodes themselves b. offline sniffing requires logging facilities on the sniffer nodes c. a wireless testbed requires no physical instrumentation (i.e. wiring) of the sensor node d. a wired testbed requires no physical instrumentation (i.e. wiring) of the sensor node Embedded Software exam April 16,